The subject matter disclosed in this application relates generally to airfoils and a method for protecting airfoil leading edges, and more specifically to a metallic leading edge strip for composite material fan blades and stator vanes that includes a structural feature that prevents domestic object damage (DOD) to downstream engine elements should portions of the leading edge strip break away from the blade or vane.
Many modern turbine engine fan blades and stator vanes are constructed of a composite laminate or molded fiber. These include the Joint Strike Fighter (JSF) F136 engine and the well-known CF6-80C2 high bypass engine. Erosion of the vane material within the harsh environment of the engine is prevented by a V-shaped protective metallic strip which is wrapped around the leading edge and covers a substantial percentage of both the suction and pressure sides of the vanes. In prior art constructions, fitting the metallic leading edge strip is difficult, principally because of the difficulty in controlling the thickness uniformity of the adhesive layer that bonds the metallic strip to the vane. The leading edge of modern airfoils is relatively sharp, and the fit up of the metallic strip to the leading edge is not exact.
Experience with composite stator vanes has shown that the thin metal strips bonded to the leading edge of the airfoil for erosion protection may become detached during engine operation. Detachment is typically due to bonding failure caused by strain mismatch between the metal strip and the composite material of the blade or vane during operation at elevated temperatures. Detachment of leading edge strips can cause unacceptable domestic object damage (DOD) to airfoils and other engine components located downstream in the engine flow path.
An important element in controlling bonding integrity is the ability of the bond material to act as a compliant layer between the composite material and the metal strip to absorb the strain mismatch. When detachment occurs, the structure of the current leading edge strips does not permit control of the size of the detached strip fragments, leading to the possibility of downstream ingestion of relatively large, irregularly-shaped metallic material and consequent engine damage.
Therefore, there exists a need to minimize detachment of protective leading edge strips from airfoil leading edges and to reduce damage to downstream engine components in the event of detachment.